Recent experiments on a 100 GHz quasioptical gyrotron have shown that for a large resonator setup [Phys. Fluids B 2, 1654 (1990)] the observed frequency up-shift between the starting current and a current of 10 A corresponds to a shift of 4-5 longitudinal modes. In this Letter it is shown that the interpretation of this frequency up-shift should involve the current-dependent electron beam voltage depression in the beam tunnel and interaction region for both the single-mode and multimode time evolution codes.